This invention relates to methods and circuits for separating a radio frequency signal of interest from a co-channel interfering signal, and to receivers which employ such methods and circuits.
In the reception of radio frequency signals, a situation is frequently encountered in which a weak signal of interest (SOI) is subject to interference by a strong interfering signal having a frequency band which encompasses that of the SOI. The interfering signal may be, for example, a jamming signal or a commercial radio or television signal. One type of signal of interest is a digital subcarrier that broadcasts digital audio beneath a commercial radio broadcast. The separation of the SOI from the undesired interfering signal under these conditions is not achievable with standard linear or frequency selective filter networks.
Various techniques such as phase and amplitude cancellation systems and steerable null antenna systems have been used to attempt to separate a signal of interest from an interfering signal. Such systems require multiple antenna apertures, accurate phase and gain matching, and/or geographic separation of the signal of interest source and the interfering signal source.
Certain conditions may provide a sample of the undesired interfering signal which can be algebraically subtracted from the composite signal to expose the desired signal of interest. This is feasible only if three conditions are met. First, the sample of undesired interfering signal must be corrected for amplitude, phase and group delay. Second, the sample must have a noise floor less than the desired signal of interest. Finally, the sample must have a make-up, in amplitude or phase, different from that of the composite signal in order to avoid cancelling the signal of interest. Where a sample of undesired interfering signal of sufficient quality is unavailable, this technique cannot be used.
The present invention seeks to provide a method and apparatus for detecting a radio frequency signal of interest in the presence of an interfering radio frequency signal when a sample of the interfering signal is unavailable, without requiring multiple antenna apertures, accurate phase and gain matching, and/or geographic separation of the signal of interest source and the interfering signal source.